In the electric arc welding of steels, it is conventional to deposit a windrow of granulated flux on the seam to be welded and then to advance an electrically energized low carbon steel electrode through the windrow to establish an arc between the end of the electrode and the edges of the seam to be welded. The arc melts these edges and the end of the electrode to form a molten weld pool. At the same time, it melts some of the granular flux which floats on top of the molten weld bead and solidifies after the molten steel in the weld pool puddle thus shaping the surface of the solidified weld bead and giving it an appropriate surface texture.
These fluxes in the past have been mixtures of various known fluxing ingredients such as: the fluorides of calcium and potassium; the oxides of aluminum, silicon, manganese, titanium, zirconium and the like; and, sometimes deoxidizers, all in carefully controlled portions selected to give: a desired solidifying temperature to the molten slag; desired slag removal characteristics; and, most importantly, desired mechanical properties to the deposited weld bead.
The various flux ingredients in powdered form are normally thoroughly intermixed and then either: fused by heating to a temperature where all of the ingredients melt and react with one another, are cooled and crushed to the desired particle size; or, agglomerated where a low melting temperature binder, such as sodium silicate, is added to the mixture and the mixture is then heated, binding the other particles in unreacted state into granules of the desired size.
Fused fluxes are more expensive to manufacture because of the greater energy requirements to melt all of the ingredients or sophisticated equipment to withstand the higher temperatures and the additional step of cooling and crushing the liquid mass. Additionally, the fused fluxes are more expensive to use because a greater amount melts during the welding. The present invention is an agglomerated flux although the principles on which the present invention excels may also apply to fused flux formulations. On the other hand, the formulations used in fused fluxes are often not usable in agglomerated fluxes.
The mechanical characteristics required of a weld bead are normally good tensile strengths and now, more importantly, high impact strengths as measured by various established recognized testing techniques such as the Charpy Impact test.
In such a test, a specimen of prescribed size is taken from the weld bead, notched and then subjected to an impact sufficient to break the specimen at the notch. The higher the energy to fracture the specimen, the higher the impact value.
One expedient adopted heretofore for increasing the notch toughness of the weld metal was the inclusion of various known potent metallic deoxidizers or alloys in the welding flux which would combine with oxygen in the weld pool and would then become part of the slag thereby decreasing the weld metal oxygen content. However, the inclusion of potent metallic deoxidizers such as aluminum or titanium in either a fused or agglomerated flux is difficult because they readily oxidize at the temperatures used to manufacture flux. Adding them after the flux has been fired and sized presents a problem of these elements settling out in the fluxes during shipment or handling. Additionally, the amount of metallic deoxidizers used must be carefully controlled as any excess will be recovered in the weld deposit and, in multi-pass welding, increase with each pass ultimately reaching a point where the amount becomes excessive with detrimental effects on the mechanical properties.
Research has shown that, as the oxygen content of the weld bead is reduced, the Charpy Impact values increase. The oxygen may be present in the weld bead in the form of oxides of iron or of any of the metals contained in the welding flux, electrode, or base metal. These oxides, if present in the weld bead, appear as microscopic particles which fail to float to the surface of the molten weld metal before it solidifies and thus remain interspersed throughout the weld metal along the grain boundaries resulting in potential low energy fracture regions in the solidified steel.
Heretofore, research has shown that in order to progressively reduce the oxygen content of the weld metal without the limitations of metallic deoxidizers, it has been necessary to progressively increase the basicity of the flux.
This phenomenon has been explored and proven time and again leading to several equations to calculate basicity and thereby estimate the result of oxygen in the weld metal and the impact values of the weld metal.
The problem has been that fluxes of high basicity generally have poor welding characteristics. These fluxes have included higher and higher amounts of the basic flux ingredients, such as calcium fluoride, calcium oxide and magnesium oxide, and less and less of the acid flux ingredients, such as silicon dioxide. They have been unable to produce quality welds in much more than an open butt joint.
It is known that low basicity fluxes in general weld at higher speeds on various joint configurations with less undercutting and slag entrapment. Also, lower basicity fluxes generally have good slag removal which allows their use in small tight (small angle) joints, deep grooves or fillets. However, such fluxes, prior to this invention, have been unable to produce welds with low levels of oxygen and high impact strength. Thus, heretofore to obtain good notch toughness of the weld bead, the welding operator has had to use high basicity fluxes and sacrifice welding speed, weld bead appearance, and general operator appeal to meet these stringent specifications.